Is it possible to access BitLocker encrypted external hard drives from other platforms, like Windows Mobile, Android, Surface, Linux, Mac OS X?
No, BitLocker is supported only by Microsoft Windows operating systems, excluding Windows Mobile and older than Windows Vista OS.
The answer is : it depends, because you mention Bitlocker which is not always what's used on external hard drive.
Two different formats
First, it seems that:
Since Windows expects one format or the other depending on how the device is plugged (say, the same SATA drive through USB or eSATA cable yield different expectations), switching back and forth causes problems on Windows (like: you can't access your data because Windows expects the other format). See for example Access BitLocker Encrypted Drive Externally - Microsoft Community and windows - Bitlocker-To-Go on fixed drive - Super User.
Now, back to the multi-platform question.
Access Bitlocker partition on other platform: yes on Linux, probably yes on others
There is a GPL-licensed software that can read a Bitlocker device through Linux FUSE or just make a decrypted copy of the volume. The FUSE mode is probably Linux-specific, but the "decrypt to a copy" mode may be more portable.
The software is named Dislocker. From its README:
Access Bitlocker To Go volume on other platforms: not so far ?
This is the case I was faced with, and noted that Dislocker could not deal with it. Reading source code, it is written to assume an encrypted Bitlocker partition, not a volume filled with Bitlocker To Go files.
In this case, the answer seems "no". Your best bet is probably to use a Windows machine to access files or create a virtual machine inside a non-Windows host.
What I did is explain the case to the person that came with the encrypted USB stick. Encryption was overkill in this case, just "company policy" had all Windows machine automatically turn any inserted USB stick into an encrypted one. The file we needed was actually not sensitive at all so he went back to his office and sent it to me via e-mail. Also, the file was a Microsoft Office file which is another matter. We finally had the job done anyway. :-)
The answer is "No, you won't be able to access a disk encrypted by Bitlocker on other OS than Windows7 and up". Why? This is speculative and can't be answered for sure, a quick shot is just "why should Microsoft support other OS for something that is usually bound to the machine itself?".
An alternative for multiplatform is http://www.truecrypt.org/
Yes, you can. There is dislocker for linux and os x. It's a command line program, open source, and until now only supports NTFS encrypted partitions up to Windows 7. Can access in read and write mode, but doesn't allow you yet to create a new encrypted partition, so you have to do it with windows.