I have seen in Windows that you can assign drive letter or it is automatically being assinged by system.

The letters that are assigned must fall any from A to Z

Once I had in total more than 26 drives (out of which many were around 13 were logical drives many Virtual CD drive using UltraISO and PowerISO)

Then it stopped allowing more device to show up and also we can not assign drive names like

AA, AB , AC, AD and so on.

Why this is not possible?

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    Because Microsoft decided that a case that covers 0.01% of their customers is not worth spending money on to allow more then 26 drives. We would only be guessing on the real reasons would be of course. – Ramhound Oct 3 '13 at 15:47
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    I believe you could mount volumes in folders once you run out of drive letters, at least. – Kale Muscarella Oct 3 '13 at 15:47
  • If you would like to mount as many (within reason), you could mount to directories (as @Huskehn suggests). – nerdwaller Oct 3 '13 at 15:49
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    I believe that's one of those questions where answers are whether unhelpful or don't answer the actual question. The unhelpful (but correct) answer would be Because it was designed like that (see Hennes' great answer below) - it answers your question, but doesn't solve your problem. The solution for your problem was provided in comments by Huskehn, but it doesn't answer your question (it's like how to overcome that?, not why isn't it possible?). If your question really is why?, then I'm not sure if SuperUser is a good place to ask. – gronostaj Oct 3 '13 at 16:57
  • Is this a theoretical question? Because if not, where are you plugging all these drives in? – TheXed Oct 3 '13 at 18:25

This is due to historical reasons. Many operating systems do not use drive letters. They either used a patch (e.g. unix like /, /usr, /mount/disk2) or a device name (e.g DF0: for floppy disk 0).

MSDOS is a CP/M clone which used drive letters. Back in those days (around 1981) you would never run into the luxury problem of needing more than a few.

The first windows versions were build on top of MSDOS and had the same limitations.

More modern windows systems do not. You can choose to mount a filesystem either under a path on an NTFS volume, or you can choose to use and old-fashioned drive letter. This part is mostly for compatibility with older programs and is has not changed. This includes no change to allow for dual letter letters.

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