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
- 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.
- External SCSI enclosures 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. 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 to 68 or 80 pin SCSI adaptor to a multi-bay enclosure could be more economical than buying adapters for individual drives. Drives too small for practical single use can be combined. You gain a cheap supply for high speed drives. This especially applies to people with access to server farms, where drives are regularly swapped out for new ones.
- A SCSI RAID array would be a very flexible, high performance, and relatively expensive external solution. Typically used with servers, these boxes have an intelligent RAID controller onboard, and interface to the host via a single cable. Other advantages are as described in the above entry on SCSI external boxes.
IDE and Flash
- For frequent use with heavy writing load, where write speed is important, try a SCSI-IDE converter with a new silent IDE harddisk (2.5" or 3.5"). IDE drives are the cheapest option in very large capacities, and may offset the cost of the converter.
- If you need very low noise, or low heat (as in a fanless Mac), but with top performance and guaranteed reliability, try a Solid State Disk (SSD) or industrial-grade Compact Flash (CF) card and adaptor. This comes at a cost compared to consumer flash devices. 
- A recent consumer CF card in an appropriate adapter should do well in a vintage machine. (Use for virtual memory is debated)
- A MicroDrive would give you the benefits of a small footprint, low power consumption, noise and heat, and fast write speeds. While not as robust against physical damage as Flash devices (ie it still has moving parts), it has none of the write-cycle limitations of consumer-grade Flash.
- A striped RAID can provide even faster performance.
- 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.