SCSI hard disk replacement options
Most classic macs use a hard disk drive (HDD or HD) as their boot device and main mass storage medium. The majority use SCSI drives of a kind no longer in production; either 50 pin 3.5" desktop drives, or the exceptionally rare 2.5" laptop SCSI drives.
There are several replacement options for defective SCSI drives. Even with a working SCSI boot drive, a second drive for extra storage is often desirable.
The choice depends on the expected use of the device, and the performance required. Note that the throughput of a faster, more modern drive is usually throttled by the slow SCSI speed on the Mac itself. Unless you are using a high-performance drive controller, it will still be limited to the host Mac's speed
Choices may be classified by
- Storage medium
- Adapter(s) required
- Use, or
50 Pin SCSI
The simplest solution of course is another 50 pin SCSI drive. Typically these are older drives now, and limited to around 4.5GB to 9GB in capacity. They may also be loud and hot server drives. However they have the advantage of plugging straight in to all 68k Macs except the Powerbooks, space permitting.
Note that there are two versions of the 50 pin connector, large and small. Macs use the larger of the two. A simple socket adapter is all that is required.
Newer SCSI drives (internal)
3.5" SCSI drives are still available, but rarely with the native 50 pin interface of older Macs. An adapter from 68 or 80 pin to 50 pin, or a SCSI card, is required. These are usually server drives: they can be noisy, but offer relatively large capacities, high performance and reliability.
Note that SAS or Serial Attached SCSI is not compatible.
External SCSI enclosure
These are available for single drives, pairs, and multiples of 7, 14 and so on. With a long enough cable, the enclosure can be hidden in a place where noise can be contained. An external drive can still be used as a boot drive, and the enclosure can easily be unplugged and connected to different Macs, including Powerbooks.
Some host 68 or 80 pin drives internally, yet interface to the host with a 50 pin cable. Even if not, a single 50 pin adapter to a multi-bay enclosure could be more economic than buying adapters for individual drives.
Drives too small for practical single use can be combined, making a cheap, high speed storage unit. This especially applies to people with access to server farms, where drives are regularly swapped out for new ones.
For many classic Macs, including all 68k Powerbooks, an external enclosure is the only simple way of connecting a CD-ROM drive.
RAID arrays of newer SCSI disks can be built internally in larger Macs, such as the tower Quadras, with the use of a SCSI card.
A dedicated external RAID array would be a very flexible, high performance, and relatively expensive solution. Typically used with servers, or in audio or video production, these boxes have an intelligent RAID controller onboard, and interface to the host via a single cable. Other advantages are as per SCSI external boxes, and overall speed remains restricted as described in the introduction.
IDE and Flash
SCSI to IDE converter
Main entry: SCSI-IDE converter
Readily available, but somewhat expensive converters. However, IDE drives are the cheapest option in very large capacities, and may offset the cost of the converter. For frequent use with heavy writing load, where write speed is important. Modern IDE disks can be near silent, and have low heat output. Ideal where a single, large boot disk is desired.
2.5" SCSI-IDE converters for Powerbooks are much harder to find, and expensive. One known source is inside Powerbooks; any SCSI drive larger than approximately 750MB is possibly a converted IDE drive. These converters can mount up to a 6GB IDE drive.
Solid State Disk
Solid State Disk (SSD) or industrial-grade Compact Flash (CF). Available as, or adapted for, IDE and SCSI, these are small, silent, have no moving parts, are shock resistant, and produce little heat. Expensive, but with top performance and guaranteed reliability, and ideal where silence or low heat are a priority. 
At much lower cost, a recent consumer CF card in an appropriate adapter should do well in a vintage machine, and give many of the same advantages. (Long term reliability, write speeds, and use for virtual memory, are debated)
While CF to SCSI adapters can be found, more commonly two adapters will be required; one CF to IDE and one IDE to SCSI, or CF-PCMCIA and PCMCIA-SCSI (See below)
A MicroDrive is a tiny (1.8") hard disk in Compact Flash format.
It has a small footprint, low power consumption, noise and heat, and fast write speeds. While not as physically robust (ie has moving parts), it has none of the write-cycle limitations of consumer-grade Flash.
Adaption to SCSI is as per Compact Flash.
- Network Attached Storage (NAS) devices provide a LAN interface to a hard disk, typically IDE/ATA or SATA. If your Mac has Ethernet and a working boot disk this is an option for extra storage. In some cases, this may be as fast or faster than a local SCSI disk. Used IDE NAS devices can be picked up fairly cheaply.
- SCSI-mounted card readers for PCMCIA (PC Card), CF, or other storage devices are under investigation, with some success reported.
- And, a more hypothetical case, if the price should not matter at all, have an UPS buffered RAM-disk and stream the data to a NAS.
If you are interested in a solid state storage solution, go to the page about using flash memory.
If want to use an IDE drive, go to the page about SCSI-IDE converter.
Always have a working backup of your important data!
- Some CF adapters mount two cards; it remains to be seen if both will be recognised in a SCSI Mac.