That was VERY useful information. kern.log is very interesting, here is a resume (full log at the end):
FAT: utf8 is not a recommended IO charset for FAT filesystems, filesystem will be case sensitive!
...
FAT: Filesystem panic (dev sda1)
clusters badly computed (321 != 314)
File system has been set read-only
followed by a lot of
FAT: Filesystem panic (dev sda1)
clusters badly computed (322 != 315)
Also fsck.vfat found some errors of the type 'cluster lenght does not match file lenght', but I'd say these are consequences of the problem, not the cause. I'm now downloading the smartmontools package and I will also mount the drive without the utf8 option and see if there is a problem. I will write again later.
Concerning moving to the Linux forum, I'd say at least Mac users may have the same problem, so how about if I start a thread with a description of the problem and link here?
Jul 24 13:46:06 localhost kernel: [17179942.420000] usb 3-3: new high speed USB device using ehci_hcd and address 2
Jul 24 13:46:06 localhost kernel: [17179942.716000] SCSI subsystem initialized
Jul 24 13:46:06 localhost kernel: [17179942.772000] Initializing USB Mass Storage driver...
Jul 24 13:46:06 localhost kernel: [17179942.776000] scsi0 : SCSI emulation for USB Mass Storage devices
Jul 24 13:46:06 localhost kernel: [17179942.776000] usb-storage: device found at 2
Jul 24 13:46:06 localhost kernel: [17179942.776000] usb-storage: waiting for device to settle before scanning
Jul 24 13:46:06 localhost kernel: [17179942.776000] usbcore: registered new driver usb-storage
Jul 24 13:46:06 localhost kernel: [17179942.776000] USB Mass Storage support registered.
Jul 24 13:46:11 localhost kernel: [17179948.012000] Vendor: Maxtor Model: 6L200P0 Rev: BAJ4
Jul 24 13:46:11 localhost kernel: [17179948.012000] Type: Direct-Access ANSI SCSI revision: 02
Jul 24 13:46:11 localhost kernel: [17179948.020000] usb-storage: device scan complete
Jul 24 13:46:11 localhost kernel: [17179948.088000] Driver 'sd' needs updating - please use bus_type methods
Jul 24 13:46:11 localhost kernel: [17179948.096000] SCSI device sda: 398297088 512-byte hdwr sectors (203928 MB)
Jul 24 13:46:11 localhost kernel: [17179948.096000] sda: assuming drive cache: write through
Jul 24 13:46:11 localhost kernel: [17179948.156000] SCSI device sda: 398297088 512-byte hdwr sectors (203928 MB)
Jul 24 13:46:11 localhost kernel: [17179948.156000] sda: assuming drive cache: write through
Jul 24 13:46:11 localhost kernel: [17179948.156000] sda: sda1
Jul 24 13:46:11 localhost kernel: [17179948.180000] sd 0:0:0:0: Attached scsi disk sda
Jul 24 13:46:11 localhost kernel: [17179948.200000] sd 0:0:0:0: Attached scsi generic sg0 type 0
Jul 24 13:46:12 localhost kernel: [17179948.752000] FAT: utf8 is not a recommended IO charset for FAT filesystems, filesystem will be case sensitive!
Jul 24 13:51:53 localhost kernel: [17180290.256000] eth0: Media Link On 100mbps full-duplex
Jul 24 13:52:03 localhost kernel: [17180299.412000] eth0: Media Link On 100mbps full-duplex
Jul 24 13:52:09 localhost kernel: [17180307.428000] eth0: no IPv6 routers present
Jul 24 13:52:35 localhost kernel: [17180333.488000] FAT: Filesystem panic (dev sda1)
Jul 24 13:52:35 localhost kernel: [17180333.488000] clusters badly computed (321 != 314)
Jul 24 13:52:35 localhost kernel: [17180333.488000] File system has been set read-only
Jul 24 13:52:35 localhost kernel: [17180333.488000] FAT: Filesystem panic (dev sda1)
Jul 24 13:52:35 localhost kernel: [17180333.488000] clusters badly computed (322 != 315)
Jul 24 13:52:36 localhost kernel: [17180333.488000] FAT: Filesystem panic (dev sda1)
Jul 24 13:52:36 localhost kernel: [17180333.488000] clusters badly computed (323 != 316)
etcetera