the maximum number of drives that can be installed depends upon how many drive bay slots your case has and how many SATA connections your motherboard has.
There can be any number of drives in Windows Explorer. The minimum number of drives is one (for the primary partition Windows is installed on).
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AC input
Hard disk drives
27 (one for each letter of the alphabet).
There's no exact limit; it depends on what kind of hardware you have to control the disks. Assuming the motherboard had six slots, and each one had a SCSI controller that supported 16 drives, you could add up to 96 hard drives! Of course, you would have trouble fitting that many in the case, and SCSI cables can only go so far.
You can have a maximum number of 26 drives / partitions in Windows 95, one for each letter of the alphabet. A: and B: are reserved for floppy drives. The rest (C: through Z:) can be used for hard drives, CD-ROM drives, tape drives, and other storage devices.
in internal bays
Only limited by the availability of free space in the extended partition.
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For internal hard drives, the highest I've seen being sold were up to 15000 RPM. Usually hard drives that come with computers pre-installed are at 7200 RPM so it's more than double that.
It depends on your computer most these days come with just one with the capability of adding at least 3 or more.