After some checking I guess I can now post my problem:
summary: WIIMC launch problems & no HDD-mounting
Loading WIIMC as homebrew from USB-loader GX results in the Wii freezing at black screen. This happens when WIIMC is on HDD or SD. Starting WIIMC via its forwarder channel tries to launch WIIMC from SD, even if WIIMC is only present on HDD. When WIIMC is booted up (from USB-loader GX, using the channel forwarder to launch WIIMC from SD), WIIMC only shows the SD contents, no HDD mounted.
Loading WIIMC via channel forwarder from System Menu or directly from HBC makes it load up as it should: WIIMC may be on the HDD and HDD is mounted on startup.
there has been a similar issue some time ago: https://gbatemp.net/threads/usb-loader-gx-not-loading-channels-and-apps-correctly.359659/
summary: WIIMC launch problems & no HDD-mounting
Loading WIIMC as homebrew from USB-loader GX results in the Wii freezing at black screen. This happens when WIIMC is on HDD or SD. Starting WIIMC via its forwarder channel tries to launch WIIMC from SD, even if WIIMC is only present on HDD. When WIIMC is booted up (from USB-loader GX, using the channel forwarder to launch WIIMC from SD), WIIMC only shows the SD contents, no HDD mounted.
Loading WIIMC via channel forwarder from System Menu or directly from HBC makes it load up as it should: WIIMC may be on the HDD and HDD is mounted on startup.
there has been a similar issue some time ago: https://gbatemp.net/threads/usb-loader-gx-not-loading-channels-and-apps-correctly.359659/