Does anyone know how I can enable
ext3 partitions on my LG Optimus V? I have already partitioned my 16GB SD card to 14.5 GB FAT32 and 500MB ext2. When using Link2SD it gives me a mount error. I am rooted but have not flashed any ROMs yet.
I am not completely certain what you mean when you say enable, so this might not answer your question.
Filesystem support is determined by the kernel, so if the kernel was not compiled with support added for a specific filesystem, you cannot use it. That being said, all android devices I have fiddled with have had support for ext2/ext3 compiled in (I think all Gingerbread devices must have support for ext2/ext3 since they are starting to use it on the eMMC flash instead of YAFFS2).
In order to check that ext2/ext3 is working correctly, I suggest you connect your device to a PC and log on via 'adb shell'. From there, create a mount-point (for example by doing 'mkdir /mnt/tmp') and try mounting manually. SDcards usually end up as /dev/mmcblkXpY, where X is the device number and Y the partition number. Try mounting by doing 'mount -t ext3 /dev/mmcblkXpY /mnt/tmp'. If that works, and you can read and write to /mnt/tmp, ext3 is working as it should.
Some cards do not support creating ext2 partition larger than 6.5 GB (I faced the same in 64GB SD card). Try Mini Tool Partition utility. Once it completes the ext2 partition format, you should not see other in the partition type space holder. It should be ext2. If it shows other, it will never work in the phone. Try reducing the size of the fat32 gradually and create ext2 after that.
In Google Play exists EzyMount, a tool with which you can mount and make accessible devices formatted with NTFS, Ext2-4, etc. for Android 2.3 and greater.