Unfortunately, my theory has been proven wrong this morning, I just had a freeze with the HDD plugged into the USB2 socket.
Hope this doesn't break the rules - here's the output from MediaInfo
General
Complete name : S:\Films\E\Ever Decreasing Circles\Series 04\00-01 New Horizons.mp4
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42 (isom/iso2/avc1/mp41)
File size : 1.02 GiB
Duration : 1 h 20 min
Overall bit rate mode : Variable
Overall bit rate : 1 808 kb/s
Encoded date : UTC 2016-11-04 13:49:54
Tagged date : UTC 2016-11-04 13:49:54
Writing application : HandBrake 0.10.5 2016021100
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : Main@L4
Format settings : CABAC / 4 Ref Frames
Format settings, CABAC : Yes
Format settings, Reference frames : 4 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 1 h 20 min
Bit rate : 1 642 kb/s
Width : 696 pixels
Height : 576 pixels
Display aspect ratio : 1.289
Original display aspect ratio : 1.290
Frame rate mode : Variable
Frame rate : 25.000 FPS
Minimum frame rate : 12.500 FPS
Maximum frame rate : 25.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.164
Stream size : 949 MiB (91%)
Writing library : x264 core 142 r2479 dd79a61
Encoding settings : cabac=1 / ref=1 / deblock=1:0:0 / analyse=0x1:0x111 / me=hex / subme=2 / psy=1 / psy_rd=1.00:0.00 / mixed_ref=0 / me_range=16 / chroma_me=1 / trellis=0 / 8x8dct=0 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=0 / threads=12 / lookahead_threads=4 / sliced_threads=0 / nr=0 / decimate=1 / interlaced=0 / bluray_compat=0 / constrained_intra=0 / bframes=3 / b_pyramid=2 / b_adapt=1 / b_bias=0 / direct=1 / weightb=1 / open_gop=0 / weightp=1 / keyint=250 / keyint_min=25 / scenecut=40 / intra_refresh=0 / rc_lookahead=10 / rc=crf / mbtree=1 / crf=20.0 / qcomp=0.60 / qpmin=0 / qpmax=69 / qpstep=4 / vbv_maxrate=20000 / vbv_bufsize=25000 / crf_max=0.0 / nal_hrd=none / filler=0 / ip_ratio=1.40 / aq=1:1.00
Encoded date : UTC 2016-11-04 13:49:54
Tagged date : UTC 2016-11-04 13:49:54
Color range : Limited
Color primaries : BT.601 PAL
Transfer characteristics : BT.709
Matrix coefficients : BT.601
Menus : 3
Codec configuration box : avcC
Audio
ID : 2
Format : AAC LC
Format/Info : Advanced Audio Codec Low Complexity
Codec ID : mp4a-40-2
Duration : 1 h 20 min
Bit rate mode : Variable
Bit rate : 160 kb/s
Channel(s) : 2 channels
Channel layout : L R
Sampling rate : 48.0 kHz
Frame rate : 46.875 FPS (1024 SPF)
Compression mode : Lossy
Stream size : 92.7 MiB (9%)
Title : Stereo / Stereo
Language : English
Default : Yes
Alternate group : 1
Encoded date : UTC 2016-11-04 13:49:54
Tagged date : UTC 2016-11-04 13:49:54
Menus : 3
Menu
ID : 3
Codec ID : text
Duration : 1 h 20 min
Language : English
Encoded date : UTC 2016-11-04 13:49:54
Tagged date : UTC 2016-11-04 13:49:54
Menu For : 1,2
00:00:00.000 : Chapter 1
00:10:46.400 : Chapter 2
00:18:53.600 : Chapter 3
00:25:26.160 : Chapter 4
00:29:14.640 : Chapter 5
00:37:40.880 : Chapter 6
00:44:58.800 : Chapter 7
00:53:46.600 : Chapter 8
01:04:47.120 : Chapter 9
01:12:43.120 : Chapter 10