I opened a ticket as I think I read the same and the fortinet tech pointed me to the following:
Please be advised that from version 5.2 Packet Capture is no longer available from the GUI. It uses flash memory on device which is not available for logging(flash-based logging) and packet capture on 5.2.X release, please read page 18 of release notes on below link.
https://info.fortinet.com/files/FortiOS/v5.00/images/build0589/fortios-5.2.0-release-notes.pdf
Fortigate is discouraging the use of flash memory on low end devices. The reasons for this is because the flash memory on devices are not designed for constant read/writes, so saving logs or capturing packets to it can degrade the disk (resulting in corrupted sectors).This feature was already removed (from GUI) on first 5.2.0 GA release so it's not just 5.2.4 which is missing this feature. The link I provided is for a reference only. The following models are affected. • FG-100D (P09340-04 or earlier) • FG-20C • FG-20C_ADSL_A • FG-200B/200B_POE (Without FSM) • FG-300C_Gen1 (P09616-04 or earlier) • FG-40C • FG-60C • FG-60C-POE • FG-60C-SFP • FG-70D • FG-60D • FG-80C/80CM (P05403-05, P05446-05) • FW-20C • FW-40C • FW-20C_ADSL_A • FW-60CX_A • FW-60C • FW-60CM (P08962-04 or later) • FW-60CX_ADSL-A • FW-60D • FW-60D-POE • FW-80CM (P05405-06 or later)
I have suggested why not write the logs and captures to an external USB drive. Just an idea. The capture tool is useful