While the majority of DEC's PDP-11 systems used UNIBUS or QBUS based peripherals for storage, some of the larger PDP-11s, along with larger machines such as the DECsystem 20 and the VAX-11/780 used MASSBUS for storage devices. As far as I can tell, the majority of these such as the RM02, RM03, and RM05 were all originally designed and manufactured by CDC. The RM05 specifically is based on the CDC 9766.

The information that I'm interested in is what range of disk packs were compatible with the RM05s? I've seen several notes that mention that RM05s should be able to take stock CDC 9766 packs instead of the DEC branded "RM05P" part. However I've also seen mentions of another compatible disk pack, the Nashua 4473. I can't find much information about this part aside from a few mentions, but assuming it was compatible this would suggest that there may have been some sort of standard and potentially other manufactures or other disk packs were compatible. Which disk packs (and their part numbers) were compatible with the DEC RM05 disk drive?

  • You mentioned the Nashua 4473, I happen to have one and came across your post while researching it. Let me know if it would be useful to you, I'm not really sure about it's value.
    – Ramzi
    Feb 14, 2022 at 13:42
  • @Ramzi I have RM05 disks that I am working on restoring and have been trying to source at least one disk pack for a little while. If you could contact me it would be rather helpful.
    – Xav101
    Feb 15, 2022 at 20:03

1 Answer 1


The RM05 is a CDC SMD drive by all mechanical/drive means, like with many of the compatible manufacturers (Ampex, Nec, Fujitsu, Siemens). Thus any of the compatible 300 MB SMD packs will fit, no matter if BASF, Nashua or Ampex or any of the device manufacturer branded ones.

Keep in mind, this is only true for the drive itself and the way it handles the disk pack. Controller and host interface as well as recording format differ greatly between various computers/manufacturers.

As long as its about using a pack, i.e. have it formated at the target machine, any pack can be used. This is different when it's about reading a 'foreign' pack, that is one written at another drive, as it may have had a different head alignment. Back in the days, service men kept all drives within standard setting, but nowadays maintenance of such drives is usually poor and not at all places kept to the same standard. Thus reading packs from other places may not always work out.

  • 1
    "Controller and host interface as well as recording format differ greatly between various computers/manufacturers" - heh, MAI Basic Four was notorious for rewriting their disk controller, otherwise CDC I think, so that bits were read/recorded backwards w.r.t. words. And you could not format packs. This made it impossible to buy ordinary CDC packs and use them, you were required to by them from MAI at a highly inflated price!
    – davidbak
    Jan 14, 2022 at 17:29
  • :)) While being to the extreme, this was common practice in wold of closed systems. Where packs could easy be exchanged between mainframes of almost all manufacturers, it gets more and more random below /360 level. P.S.: at some point I may have to tap your knowledge - that is, if I ever come around to get one of my MAI running again.
    – Raffzahn
    Jan 14, 2022 at 17:37
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    (stupid spell check I meant REWIRE not REWRITE but of course you figured that out)
    – davidbak
    Jan 14, 2022 at 18:01

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