On 11 Feb 2009, at 01:11, Bob C. wrote:
> > Covering the hole is only a short-term solution. The magnetic > properties of > the two disk types are slightly different, so long-term storage of > 800K info > on a 1.4MB disk is not recommended. > > Some people have good luck with it, others not so much. Your > mileage may > vary. :-) > > IIRC (it has been a while!) the Double Density and High Density 5.25" disks (as used in PCs) had different magnetic properties, and there was also no "HD hole", so the disk format was purely determined by the software - formatting a HD disk as DD was possible, but unreliable, but DD formatted as HD wouldn't work. Also, having been formatted wrongly, it was difficult to fix - sometimes a re-format of a HD disk back to HD would still be unreliable. The DD and HD 3.5" disks had the same magnetic properties, but were formatted in different ways. The use of an additional hole (the same size as the write-protect hole, but further down the disk) to determine DD and HD meant that the format could be determined by the hardware (the Mac correctly read the disk hardware, and then provided options depending on the disk inserted - 400k or 800k if it was DD disk; 1.44Mb (DOS or Mac) if it was HD. MS-DOS, on the other hand, having had to cope with 5.25" disks with no hardware disk typing, would format based on the magnetic type first, and only then bother to check the hardware. This meant that "format a:" would work if a DD 3.5" disk was inserted, but would fail if an HD disk was inserted - DOS tried to format the disk as DD, and then noticed the HD hole. Hence the use of "format a: /n:9" to force it into 9 track (HD) style, and yet another piece of obscure arcana required just to format a disk. But I digress...) Hence, the use of "tape over the hole" to allow a HD disk to be formatted as DD (and also the special hole punch that could be used to turn DD disks into HD ones. The main problem with the hole punch was that it could introduce tiny plastic shards into the disk area, so was not recommended for important data (but was OK for daily stuff, particularly as HD disks cost twice as much as DD (or roughly the same as the cost of 100 blank CD-R today for a box of 10 HD back in 1990!) The Apple Superdrive could read and write 400k (MFS and HFS), 800k (HFS), 1.44Mb MS-DOS, 1.44Mb PRODOS and 1.44Mb Mac (HFS) disks. However, additional software (Dayna DOSMounter, or Apple PC Exchange) needed to be bought to allow the MS-DOS disks to be used. Apple PC Exchange was a separate purchase for System 7.0, but was included with the Performa machines (7.0 & 7.1P), and then (I think) with System 7.1 onwards. regards, Alasdair --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
