UPDATE 17th April 2013: Transcend 2 GB MLC works fine as well. Amazon link.

Juniper clue tells us the following:

“The RE5 can be easily upgraded. The CF is in a hotswap socket on top of the board and can be changed easily. SanDisk SDCJF-2048 (Standard CF) are confirmed to work with RE5.0. Transcend Industrial CF Ultra TS1GCF100I (1GB) are confirmed to work with RE5.0. The CF card may need to be zeroed out before use.”

Since a 2 GB CF was kinda hard to find these days I purchased a SanDisk Ultra 4 GB CF instead (model no. SDCFH-004G-U46) – Amazon link. The CF will get detected fine by JunOS and copying everything over from the harddrive via ‘request system snapshot partition’ works as well. The only problem is that the router will remove the CF from the boot list every time you boot up, effectively making it pointless to have a CF card installed. :-)  The reason is:

ad0: FAILURE – READ status=51<READY,DSC,ERROR> error=10<NID_NOT_FOUND> LBA=7813119

Basically this is just a cosmetic error, but the router doesn’t recognize it as such and thinks your CF is broken, which it isn’t. You will probably also see:

ad0: FAILURE – SETFEATURES 0x85 status=41<READY,ERROR> error=4

Which is ok and doesn’t cause the CF to get removed from the boot list. You can find a good explanation of both errors here. Anyway, I ended up using a SanDisk Ultra 2 GB as recommended: Amazon link. Not sure if 4 GB CFs won’t work at all in the RE-5.0 or just this particular SanDisk model. An indication that only the SanDisk 4 GB causes trouble could be that when you google for the above NID_NOT_FOUND error only the SanDisk 4 GB model is being mentioned in several mailing lists posts etc.