My impression is that in order of performance it goes
MFM -> DSSI
-> SDI and that the vS3500 was considered a performance machine,
leading to the 3520 and '40. (going out on a limb now) I
would actually be surprised if DEC ever shipped a vS3500 with
MFM drives. Perhaps TMaC's system was customised by a prior user?
There was a low-end VS3500 that shipped with an RD5x drive: it
was called the VAXstation 3200. It came in a BA23 box IIRC.
Anyone who considered a VS3500 a performance machine (even when
it came out) was on something or in someone's pay or utterly
clueless :-)
Antonio
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Antonio Carlini arcarlini(a)iee.org