Yesterday I installed a custom rom on my Samsung Galaxy S2. Before I had ICS 4.0.3 installed. Now my external sdcard does not mount automatically on the phone. How can I get this working?
A lot of sdcards are formatted as ExFAT (extended FAT32 - which is better than FAT32 in a number of aspects for flash media, including support for larger than 4GB files). Unfortunately, this is a licensed and proprietary technology and custom ROMs can't legally include support. If you have a card reader for your computer, you can use that to back up the contents of your sdcard. Then you can right click on the drive in My Computer and select format. Select FAT32 and let it do its thing. When it is finished, copy the contents back to the drive and put it back in your phone.
If you don't have access to a card reader, then you will either need to use another phone to back up your content on the card, or possibly, see if you can still enable USB Mass Storage in recovery (most likely under the Mounts and Storage menu depending on the recovery used). If you can't do either, then you can just format the sdcard from within recovery, which will put the filesystem to standard FAT32 and make it readable in your custom ROM, although, this will erase all the content on the card.