Depending on many factors ranging from OS to the mobo specs, one could fit as many as four different PHYSICAL drives in any given 'modern' towers.

- PSU must have the required feeding rails.

- IDE slots (be it ATA or better) should have their dedicated master_slave configuration accordingly.

- The conventional enumeration process should detect any 'settings & hardwares' as long as the mobo specs are within their limits.

I once had an old PC with three drives in it (most were also partitionned with logical schemas, btw) and a single CD reader tied with the 2nd IDE cable as slave, and everything was running smooth and i could access all as long as the BIOS was detecting any on boot.

More important is the OS limits; old systems couldn't tackle any drives above the 40Gigs size and even then, some mobos wouldn't stick up with bigger drives than 12Gigs. Besides, that the main OS ***must*** initialize from a primary partition (or able to dual-boot with another if one setup has such capacity through external programs)... the physical limit on drives is more about how many gigs must be allocated than the maximum number of four 'items'.

One thing to note though is that in certain high-end 'monster_machines' (servers, comes to mind) the 4 max rule doesn't really apply.

But we're talkin' personal PC, right?

Hold it, there's some technicalities 'definition' that should be straigthened up i guess.

Available Buses, ScSi & SATA connectors, IDE(s) controller, number of physical slots in the casing itself... all more important than the next.

Like i said, the mobo dictates how & why the HDs array deploy and if the Bios doesn't detect the 'supposedly' compatible elements installed there's nothing else anyone can do but either get the adequate mobo specs or simply forget adding what simply can't.

The 500Gigs Sata i got along with the pretty recent gear (Acer 5640, boosted & adapted, manufactured just this January) i bought is on IDE-1 (bus_0, master) and the Blue_Ray DVD_CD_SCSI is using the second IDE (bus_2) -- considering the Raid array could still be there if needed eventually.

And since, someone brought up the 'organized' settings...

C_D_E_F_G_H_I

(It's been like this for years, the only things that changed is how the physical devices are fitted into the tower)

H_ is always the CD_DVD stuff.

I_ was an Iomega zip drive before but now, it's simply a virtual slot locked as 4.6+ to copy_move DVD folders as a whole and straight burning.

C_ core OS, either as a full drive or partitioned once the usual hidden OS stuff is left on its slam refer tag space.

DEFG_ were created off a second drive, but not this time around.

And since, there is a firewire_1394 too, i suppose i could also go the EXTERNAL way which counts as any extra drives also.

J_ is an onlooker for USB sticks (8 total in the tower, 4 backs filled with key/mouse/audio/printer, 4 front as often as necessary) -- that one used to be a CD rewriter slaved on IDE2.

Daemon received it's usual 4 (KLMN). Mostly temp Isos and testing phases for compiler works. C++ or VB stiffers.

OPQR_ are carding callers once snapped in.

The essential are named... Core (140), Data(43,8), Extra(18,6), Filer(20,8), Games(227 **GC2, btw)

D_ata; gets all & each personal files - no programs, all graphics reference stuff and right now, the GalCiv2 mods i'm developping.

E_xtra; Audio, Video, Drawing, Coders, 3D... programs only.

F_iler; All transitions... like Downloads, Uploads, a WRITE2CD & a WRITE2DVD drop zone folders growing and burnt out as often as needed. FTP searches, ImageShack files, Unzipped temporary, ISO, etc.

Of course, this is rather "unique" as everyone may have a totally different way of setting their systems.

I have 4 hard disks in my computer, 3 PATA and 1 SATA, plus a PATA dvd burner and one FDD. Also have an external hdd.

Something from the OP that no one addressed -- you may have a second copy of a program on the old drive, but if you don't re-install it, it may not work properly.  Some programs are pretty sensitive about this, some aren't.  I recommend reinstalling, copying any data over if you need it, and then reclaiming the space from the old drive.

Also, if you connect your old hard drive to the same IDE channel as the CD/DVD drive, the performance will be pretty bad.  The IDE channel operates at the speed of the slower device for moving data.